Men with recent myocardial infarction (MI) or angina, known to be at high risk for sudden coronary death, were identified over a 4-year period from a population of about 120,000 men aged 35 to 74, enrolled in the Health Insurance Plan of Greater New York (HIP). Over 2,000 such men received a special baseline examination at which information pertinent to development of prognostic data was collected according to a standard protocol. At these examinations, and at several subsequent sessions, the men were monitored for one hour with a single ECG lead recorded on a tape cassette. The ECG tapes were processed by computer and all abnormal beats written out on ECG paper; trained readers and physicians reviewed these write-outs to identify all ventricular premature beats (VPB) and characterize other features of ventricular ectopic activity. The monitored men are being followed for mortality, and information from next of kin on circumstances of death supplements all medical record abstracts. First analyses of survival have involved 1,739 men with MI prior to baseline who had been followed for mortality for periods up to 4 years (average 24.4 months). Multivariate methods which take into account other important prognostic variables establish that the presence of complex VPB (R on T, runs of two or more, multiform or bigeminal VPB) in the monitoring hour is associated with a risk of sudden coronary death that is three times that of the men free of complex VPB. Complex VPB make an independent contribution to risk of death, and the increased risk has persisted over the length of this observation period.